The 6-Digit Hydrologic Units, a component of the Watershed Boundary Dataset (WBD), depict the third level hydrologic subdivision of the United States. A 6-digit hydrologic unit may subdivide a 4-digit hyrologic unit or may be equivalent. There are 401 6-digit hydrololgic units with an average size of 10,486 square miles (27,159 sq. km.).